I never gave it much thought. But I guess I once tried to find the chord that sounded most right. And if it's played in G as the first chord my best bet is open D string, 2nd on G, 1st on B and 3rd on high E.
But that might be just plain wrong
Cool video btw!

It's interesting to hear that they've digitized the entire set of multi-tracks...though I suppose it's important enough that it makes perfect sense. There's a guy on this board who lives in Scotland, who teaches audio at the college level. Sgt. Peppers and other albums have been made accessible for students. They can't save their own mixes, but it's a great source to work with. Very generous of the boys, really.